M1 Carbine What do the letters mean?

I have a M1 Carbine that has a forearm band with the letters JM on the right side and the letters CM on the bottom of the bayonet lug. The spot between the safety lever and the mag release on the bottom of the trigger assembly in front of the trigger guard is an X.
The receiver is a Winchester and I was told that this gun probably went through the arsenal once.